Читать книгу Na-ion Batteries - Laure Monconduit - Страница 36

As mentioned above, P2-type single 3d transition metal systems are found in only P2-Na_2/3VO₂, P2-Na_0.6Mn_0.9O₂, P’2-Na_2/3MnO₂ and P2-Na_2/3CoO₂. Since Na_xMnO₂ delivers the largest reversible capacities of >200 mAh g⁻¹ among single 3d transition metal P2-type systems (Kumakura et al. 2016), various solid solutions are synthesized based on P’2-Na_2/3MnO₂ and P2-Na_2/3CoO₂ as parent oxides and their Na (de)intercalation properties have been extensively investigated in Na cells.

A solid solution between P’2-Na_2/3MnO₂ and P2-Na_2/3CoO₂ is crystallized in any ratio of Mn/Co (Paulsen and Dahn 1999; Wang et al. 2013). From the late 1990s to the early 2000s, the solid solution was studied as positive electrode materials for Li-ion batteries after Li⁺/Na⁺ exchange (Paulsen and Dahn 1999; Lu and Dahn 2001b; Eriksson et al. 2003). Na intercalation properties of the sodium solid solution were first reported in P2-Na_2/3Mn_1/3Co_2/3O₂ by Carlier et al. (2011). P2-Na_2/3Mn_1/3Co_2/3O₂ delivers a reversible capacity of 112 mAh g⁻¹ in the range of 1.5–4.0 V, as shown in Figure 1.14. The voltage profiles of P2-Na_2/3Mn_1/3Co_2/3O₂ are relatively smooth except for the voltage jump at x = 2/3 and 1/2 in P2-Na_xMn_1/3Co_2/3O₂ compared to the end-members of P’2-Na_2/3MnO₂ and P2-Na_2/3CoO₂, suggesting that the phase transitions and Na⁺/vacancy ordering in the Mn/Co-disordered phase are of less significance (Beck et al. 2014). According to XAS data of Na_2/3Mn_1/3Co_2/3O₂ during charge/discharge in Na cells (Cheng et al. 2014), LS Co³⁺ is oxidized into LS Co⁴⁺ by Na extraction and HS Mn⁴⁺ is maintained during charging to 4.0 V. On discharge to 2.5 V, LS Co⁴⁺ is reversibly reduced into LS Co³⁺. P2-Na_2/3Mn_1/3Co_2/3O₂ can accommodate more Na in the structure and both HS Mn⁴⁺ and LS Co⁴⁺ were reported to reduce into HS Mn³⁺ and HS Co²⁺, respectively, with major modification of hybridization between M 3d and O 2p orbitals by Na insertion on discharge below 2.5 V. The electronic structures significantly change with the formation of Jahn–Teller active Mn³⁺ and the reduction from LS-Co³⁺ to HS-Co²⁺ on discharge, resulting in much lower voltage than P2-Na_2/3CoO₂ below 2.5 V in the Na cells (Cheng et al. 2014; Watanabe et al. 2019). The extra 0.16 Na insertion into Na_2/3Mn_1/3Co_2/3O₂ and the large reversible capacity on discharging from 2.5 to 1.25 V is attractive, but Na compensation is necessary for the practical use. Even for P2-Na_2/3[Mn,Co,Ni]O₂, the low working voltage and indispensableness of external Na compensation are drawbacks.